Conventional SAW filters utilize a single input interdigital transducer and a single output interdigital transducer formed on the surface of a piezoelectric substrate. The input transducer converts an input electrical signal to a SAW, and the SAW propagates along the surface of the substrate to the output transducer. The output transducer then converts the SAW back into an output electrical signal. The input transducer and the output transducer thus form a SAW filter characterized by a single channel across which the SAW propagates, from the input transducer to the output transducer. However, such a SAW filter has very limited frequency tuning range, and the frequency response shape is relatively fixed.
One improvement over the conventional SAW filters is to fabricate a SAW filter producing a COMB frequency response (which as used in this description and in the claims, a COMB frequency response or a COMB response indicating, as is known to those of ordinary skill in the art, a frequency response that consists of a series of spikes that gives the appearance of a hair comb) on a voltage controlled velocity tunable piezoelectric substrate. The input and output transducer fingers of the SAW filter are periodically withdrawn to create a COMB frequency response respectively. Varying the electric field across each transducer varies the SAW propagation velocity in each transducer, thus creating relative shifts of the frequency peaks in each COMB response. Cascading the input and output COMB responses of the input and output transducers results in cancellations of the input and output peaks at offset frequencies and enhancements of the input and output peaks at the same frequency. A single frequency peak response can be scanned across the operating frequency range by strategically biasing the input and output transducers.
A SAW filter producing a COMB frequency response fabricated on a voltage controlled velocity tunable substrate can multiply the tuning range. However, this approach results in crude filter properties in terms of filter rejection, insertion loss, and fixed pass band shape.